1. Field of the Invention
The present invention relates to a combustion process, applicable more particularly to the calcining of an ore-based material, especially to the manufacture of cement, in which process a material is heated in contact with a heat source essentially created by a flame produced by at least one fuel and at least one oxidizer. This calcining process is integrated into a cement preparation process. The invention also relates to the use of the combustion process to heat a charge, whether for melting a metal, for maintaining it at temperature, for the destruction of waste, etc.
2. Description of the Related Art
The manufacture of cement involves the manufacture of an intermediate product called xe2x80x9cclinkerxe2x80x9d. The clinker is a product which is obtained by firing an ore-based material, especially clay and limestone. The material in powder form may be delivered to a rotary kiln either in dry form (dry process) or in the form of a water-based slurry (wet process). The composition of the clinker is in general carefully controlled so as to obtain the desired proportions of the various ore materials and especially calcium carbonate, silica, alumina, iron oxide and magnesium carbonate. After the kiln has been charged, the precursor material for the manufacture of clinker firstly undergoes a drying step and a heating step. Next, this material undergoes a calcining step in which the carbonates of the various ores are converted to the oxide of these ores by removal of carbon dioxide. Since the temperatures are still high, the ores thus obtained react together chemically to essentially produce calcium silicates and calcium aluminates. The latter process is also called xe2x80x9cclinkeringxe2x80x9d and is carried out in the hot zone of a rotary kiln. The resulting clinker is then cooled and pulverized, and then mixed with additional ingredients in order to form a cement such as Portland-type cement.
The processes for manufacturing cements have many similarities and the essential differences between these various processes reside essentially in the method used for drying, preheating or calcining the clinker precursor. As a general rule in all these systems, the clinker manufacturing process is largely the same, that is to say a process in which a rotary kiln is used, the clinker precursors move down along the kiln under gravity while the hot gases are circulated as a countercurrent from a zone in which combustion has been carried out.
It is known, for example from U.S. Pat. No. 5,572,938, that the use of oxygen in rotary kilns for clinker manufacture make it possible to increase the production of clinker by essentially improving the combustion usually carried out using combustion in air. However, until now these techniques have not been very well controlled and often represent a substantial increase in the production costs for the manufacturer.
Various cement manufacturing techniques are described, especially in patents U.S. Pat. Nos. 3,302,938, 3,404,199 and 3,925,091, the descriptions of which are incorporated into the present application by reference.
Other processes in which oxygen is also used in the manufacture of cement are described in patents U.S. Pat. Nos. 5,007,823 and 5,580,237.
In general, clinker manufacturers try to incorporate into their kiln, as fuels, so as to reduce the production costs, fuels which have the property of burning relatively poorly, as well as products of low combustibility which have a low net calorific value (NCV). In general, they seek to use all kinds of waste of relatively low combustibility for which they may especially receive premiums for the destruction of said waste. In fact, the clinker manufacturing process consumes a great deal of energy, particularly because the reaction of decarbonizing the calcium carbonate in the clinker manufacturing operation is a very endothermic reaction and therefore consumes a great deal of energy. p The usual fuels which burn easily in rotary kilns for clinker manufacture are coal, heavy fuel oils and natural gas. These fuels have a net calorific value (NCV) with a value of between 30 and 45xc3x97106 joules/kg. The heavy fuel oils may be preheated and atomized into droplets having a size of less than 200 microns with a fraction of their mass converted into droplets having a diameter of less than 50 microns. The smallest droplets rapidly evaporate and thus allow the flame to be ignited close to the end of the burner.
In the same way, the coal particles are pulverized with a size distribution of between 10 and 200 microns. The rapid and stable ignition of the combustion is improved by controlling the size but also by the combustible volatile material released by the particles when they are heated. However, cement manufacturers continue to make efforts to reduce the cost of the fuels used in the production of clinker and at the present time try especially to burn liquid or solid waste of low combustibilities and a net calorific value (NCV) often less than 15xc3x97106 joules/kg. However, these poor fuels often have a water content of greater than 20% by mass, or a large particle size (for example 75% of the mass consisting of particles or droplets having a size of greater than 200 microns).
The use of these fuels which are difficult to burn leads to a number of problems in the combustion zone and in particular in the calcining zone of the rotary kilns used for manufacturing clinker, and especially unstable ignition of the flame and excessively low degrees of combustion, thereby generating uncontrolled concentrations of carbon monoxide, emissions of hydrocarbons into the gases emanating from these kilns and unacceptable levels of unburned material in the ash, particularly unburned material in the gases emanating from the kiln, with consequently reductions in productivity, unless additional amounts of fuel are added so as to compensate for the deleterious effects of these poor fuels.
The problem at the basis of the invention stems from the observation by the inventors that the fuel injected into the kiln and especially the fuels having a low net calorific value were unable to contribute to the combustion before having traveled rather a long distance inside the rotary kiln. If the distance traveled in the kiln is too short, the combustion is of poor quality.
The combustion process according to the invention is distinguished in that the flame comprises a primary combustion zone created by the combustion of a first fuel and of a first oxidizer, this primary zone being located near the points of injection of the first oxidizer and of the first fuel, as well as a secondary combustion zone located downstream of the primary zone, for the combustion of a second fuel and of a second oxidizer, the second fuel being preheated by flowing through or near the primary zone of the flame.
Preferably, the distance that the second fuel flows in contact with the flame of the primary zone will be sufficient for at least. some of the second fuel to have been preheated to a temperature of at least approximately 400xc2x0 C. preferably approximately 600xc2x0 C. and even more preferably 800xc2x0 C.
According to a preferred mode of implementing the invention, it has been found that when the distance that the second fuel flows in contact with the flame was covered under conditions such that the temperature of this second fuel was substantially about 1000xc2x0 C. at least when the latter arrived in the second combustion zone, the combustion of the second fuel in this second zone was carried out properly, resulting in a reduction in the NOx content and in the amount of unburned material in the smoke.
Preferably, the secondary fuel will be a fuel whose net calorific value (NCV) will be less that 15xc3x97106 joules/kg. According to one embodiment of the invention, the secondary fuel may be a fuel whose water content by mass will be greater than or equal to approximately 20% and less than or equal to approximately 95%, preferably less than or equal to 70%. According to another embodiment, the secondary fuel will contain ash in a mass proportion of greater than 20%.
Of course, according to the invention, it is possible to use a secondary fuel or a mixture of secondary fuels (chosen especially from those mentioned above) as well as a mixture of one or more of these secondary fuels with another fuel such as the primary fuels mentioned in the context of the present description and especially those having a net calorific value (NCV) greater than 30xc3x97106 joules/kg. According to one mode of implementing the invention, the ignition distance defined as being the distance between the end of injection of the oxidizers and fuels and the start of the combustion zone will be less than 2 m, preferably less than approximately 1 m.
As a general rule, it will be considered that the primary flame zone is largely over when more than approximately 90% of the primary oxidizer has reacted with the primary fuel.
In general, the energy of the primary flame will be as low as possible and will represent at most 30% and Preferably at most 15% of the total energy provided by the flame. Preferably, the energy of the primary flame will represent between approximately 1% and 10% of the total energy provided by the flame, this primary flame preferably comprising a zone with as high as possible a temperature, so as to raise the temperature of the secondary fuel in contact with it as rapidly as possible.
According to another embodiment of the invention, the primary fuel will be a fuel preferably having an NCV of greater than 30xc3x97106 joules/kg, that is to say a fuel which ignites easily. However, it will be possible to mix this high-quality fuel with a fuel having a low net calorific value or a fuel of poor ignitability such as those fuels defined above, in proportions such that, however, a primary flame having the required temperature qualities, and especially having a temperature preferably greater than 800xc2x0 C. and more preferably greater than 1000xc2x0 C. is obtained. The primary oxidizer will be an oxidizer which will contain more than 21% oxygen, preferably more than 35% oxygen, more preferably more than 50% oxygen and even more preferably will be industrially pure oxygen, i.e. oxygen comprising more than approximately 88% oxygen by volume, such as the oxygen produced by systems for producing oxygen by adsorption, such as VSA (Vacuum Swing Adsorption) systems, and may also be formed by oxygen of cryogenic quality, i.e. oxygen having a purify often greater than 98%, optionally pure or as a mixture with air.
The secondary fuel has already been described above, whereas the secondary oxidizer will preferably be air, particularly the air which is normally used in the burner fitted into cement kilns (also called primary air and/or secondary air).